Photoflash lamp array circuit board having radiation sensitive fuse elements

ABSTRACT

Improved fuse elements used with the circuit board to provide more reliable sequential firing in a flash lamp array. The circuit board includes a lamp firing circuitry along with radiation switches connected in said circuitry and more reliable fuse elements cooperate with said radiation switches in the circuit operation. The improved fuse elements are constructed as an integral part of the lamp firing circuit pattern deposited on the circuit board at circuit locations wherein the underlying circuit board substrate has a thickness less than the substrate thickness elsewhere to enhance melting or thermal decomposition of the circuit board substrate.

BACKGROUND OF THE INVENTION

Radiation sensitive fuse elements for use with a circuit board toprovide more reliable sequential firing in the flash lamp array arealready known. For example, U.S. Pat. No. 4,133,631, assigned to theassignee of the present invention, describes a circuit boardconfiguration which includes such fuse elements being deposited atcircuit locations wherein the underlying substrate has a thickness lessthan the substrate thickness elsewhere to produce holes in the circuitboard by melting or thermal decomposition when activated by radiationbeing emitted from adjacent flash lamps. Said fuse elements aredeposited on the underlying circuit board substrate and comprise atleast 20% or more of a radiation absorbent solid particulate such ascarbon black mixed with conventional liquid adhesive which hardens to asolid at ordinary temperatures. When this fuse material composition isdeposited directly on the circuit pattern and allowed to harden atlocations on the circuit board that have been recessed to enhancelocalized melting or decomposition of the underlying substrate, the fuseactuation produces a hole through the entire composite medium.Interruption of the circuit path is obtained in this manner to theactivating flash lamp which can be disposed in a branch circuit alongwith a fuse element and thereby disrupt electrical connection to thealready fired flash lamp.

Another type known radiation-sensitive fuse element construction for usein the same general manner is described in U.S. Pat. No. 4,154,569. Thistype fuse element construction utilizes a strip-like element whichoverlies a reduced thickness area of the circuit board member and whichcan be constructed from a polymer film deposited on the circuit boardsubstrate which is thereafter perforated to help isolate the absorbedthermal energy in said strip element for more reliable thermaldecomposition of the entire fuse. The circuit board pattern on whichsaid strip elements are deposited is said to be formed of metal, forexample silver, nickel, tin, copper or other readily conductingmaterial, for example graphite. The circuit board support for theelectrical circuit is also said to be pigmented in order to improve theheat absorbing capacity of the overall construction and soluble dyes canalso be incorporated with the thermoplastic synthetic resin compositionof said dielectric support for this purpose. Additionally, a dye isrecommended for incorporation in the polymer material forming thestrip-like fuse element to further improve radiation absorption in theoverall construction.

Still other issued U.S. Pat. Nos. 3,968,056 and 3,988,647, which arealso assigned to the present assignee, describe a circuit boardconstruction which can be used with a photoflash array wherein aparticular type electrically conductive ink provides the circuitpattern. Specifically, said conductive ink comprises an organic resinmatrix having sufficient particulated electrically conductive materialdispersed therein to form point by point electrical contact when thedeposited liquid ink is cured to the solid state. In a preferredembodiment, a liquid ink composition is radiation curable to provide asolid coating having a resistivity of less than 10 ohm-centimeters whencured. The particulated electrically conductive material providing theelectrical conductivity in said circuit pattern can be selected from aparticulated electrically conductive metal and/or a particulatedelectrically conductive metal containing material which can furthercontain up to approximately 15% by weight of said particulatedelectrically conductive material in flake-like form with an aspect ratioof diameter to thickness greater than 20. The content of particulatedelectrically conductive material in said preferred embodiment is atleast about 40% and not more than about 90% by weight with saidparticulated electrically conductive material preferably being in theform of metal-coated glass spheres.

SUMMARY OF THE INVENTION

Lower intensity flash lamps are now being used for cost and energysaving considerations which create a need for fuse elements in the abovetype photoflash array that interrupt the circuit path reliably with alesser amount of radiation being emitted by such flask lamps.Accordingly, it is a primary object of the present invention to providean improved fuse element construction in a circuit board of this typehaving both radiation sensitive switching elements and radiationsensitive fuse elements as part of the circuit pattern for cooperationtherebetween when the associated lamps are flashed. Another object ofthe present invention is to provide fuse means which more reliablyinterrupts the circuit path when exposed to radiation from a cooperatingflash lamp to thermally melt or decompose the underlying substrate andcreate a hole or opening for permanent disruption of the electricalconnection. In accordance with said objects, the present improvement isaccomplished with a circuit board pattern comprising an organic resinmatrix having particulated electrically conductive material and aradiation absorbing material dispersed therein with said circuit patternfurther including radiation sensitive fuse elements located where saiddielectric substrate of the underlying circuit board member has areduced thickness relative to the remaining thickness of said dielectricsubstrate and with said fuse elements being formed by providing a morenarrow width of said circuit pattern at the fuse locations than theremaining width of said circuit pattern. The preferred radiationabsorbing material dispersed in the circuit pattern is an organic dyedissolved in the thermoplastic polymer material. As previouslyindicated, the particulated electrically conductive materialincorporated in said circuit pattern comprises metal coated glassspheres and the particulated electrically conductive material canfurther contain up to 15% by weight of said particulated electricallyconductive material being metal flakes.

The particular circuit pattern material employed in the presentinvention, which is fully described in the aforementioned U.S. Pat. Nos.3,968,056 and 3,988,647 patents hence need only be illustrated herein,serves an important role in accomplishing more reliable interruption ofthe circuit path when the present fused elements forming part of saidcircuit pattern are actuated. Electrical conductivity in said circuitpattern is solely attributable to physical contact between theelectrically conductive filler particles so that physical separationtherebetween effectively interrupts the circuit path. It is thereby notrequired that the fuse element portions of said circuit pattern becompletely decomposed to form an opening entirely across the width ofthe circuit pattern since a lesser physical separation such as multiplecracks in the circuit pattern at the fuse locations raises thedielectric breakdown voltage of the opened fuse elements sufficiently toblock a subsequently applied firing pulse. Such fuse operationrepresents an improvement compared to that taking place in theaforementioned U.S. Pat. No. 4,154,569 patent wherein the circuitpattern can be entirely metal thereby requiring thermal decomposition ofthe entire circuit pattern along with the underlying dielectricsubstrate at the fuse locations. In further explanation, the presentfuse elements operate effectively to interrupt the circuit path when theunderlying substrate supporting the circuit pattern is not totallydestroyed so long as sufficient thermal decomposition of the circuitpattern takes place at the fuse locations to block a subsequentlyapplied firing pulse.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of an otherwise known multiple flash lampcircuit board having the lamp firing circuit pattern deposited on thedielectric substrate and radiation sensitive switching devices connectedin said circuitry for sequential firing of a plurality of flash lampsconnected thereto which has been modified in accordance with the presentinvention; and

FIG. 2 is an electrical schematic diagram illustrating the circuit pathfor one group of four flash lamps being sequentially fired by thecircuit board configuration in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the preferred circuit board embodiment depicted in FIG. 1, bothradiation sensitive switches and radiation sensitive fuse elements aredeposited on the circuit pattern adjacent flash lamp locations forsimultaneous actuation of each adjoining switch and fuse element byradiation being emitted from an adjacent flash lamp. Accordingly, thecircuit arrangement of FIG. 1 is generally the same as disclosed in theaforementioned U.S. Pat. No. 4,133,631 patent which includes a circuitboard member 10, in the form of a dielectric substrate constructed witha thermoplastic organic polymer, having connector tabs 12 and 14 locatedat each end for insertion of said circuit board member into anassociated camera socket (not shown). The camera socket has contacts forconductor line 16, 18 and 20 of the lamp firing circuit pattern toconnect four flash lamps in parallel between conductive lines 16 and 18when connector tab 12 has been inserted in the camera socket.Correspondingly, a second group of four lamps is connected betweenconductor lines 18 and 20 when connector tab 14 has been inserted in thecamera socket. While an entire illustrative circuit board is shown,there is need to discuss only a single group of four flash lamps sincethe operation of the second group of four flash lamps is essentially themirror image of the first group. Conductor lines 16 and 20 areinterrupted by radiation switches 22 to prevent the flash lamps in agroup from all being fired together on release of the camera shutter. Ascan be noted, the switches 22 are each arranged in a region of a flashlamp termination 24 so that the heat and light released when the flashlamp is fired operates to convert the respective switch from anon-conductive to an electrically conductive state, thus establishing acurrent path to the next lamp to be flashed. Accordingly, when theparticular flash lamp in the circuit is flashed, then the adjacentradiation switch is actuated providing a current path to the next lampto be flashed. Each of the radiation switches is serially connectedbetween the branch circuits connecting the lamps in a parallel circuitrelationship as better shown by the schematic diagram in FIG. 2. Each ofsaid radiation switches employ a material exhibiting an open circuit orhigh resistance condition and with said resistance thereafter becomingzero or a low value when said material absorbs radiation and/or heatfrom the adjoining flash lamp. Individual fuse elements 26 are connectedserially to each of the flash lamps in the branch circuits at locationsproximate to the flash lamp terminations 24 so as to also be actuated bythe heat and light released when the flash lamp is flashed. Circularshaped indentations 28 (shown in phantom) surrounding each of said fuseelements 26 have been formed on the opposite side of the circuit boardmember to provide a lesser thickness than exists elsewhere on thecircuit board and thereby assist interruption of the circuit path fromconductor 18 to the already fired flash lamp. As can be noted, each ofsaid fuse elements simply comprises a more narrow width of the circuitpattern at the fuse locations than the remaining width of said currentpattern elsewhere. When the particular flash lamp adjacent to a givenfuse element is flashed, the radiation and/or heat created thereby isabsorbed by the circuit pattern at the fuse location whereupon the morenarrow fuse element portions of said circuit pattern become sufficientlydecomposed to disrupt the electrical connection to the already firedflash lamp. As previously indicated, such interruption of a circuit pathis accomplished by physical separation of the electrically conductivefiller particles in the circuit pattern which can simply produce aseries of cracks in the circuit pattern itself at the fuse locations.There can also be additional thermal decomposition whereby theunderlying reduced thickness portion of the circuit board member eitherthermally melts or decomposes to create an actual hole or opening ateach fuse location.

A more detailed description of the sequential firing operation for thecircuitry of a four-lamp group in the above described circuit board canbe provided in connection with the schematic drawing shown in FIG. 2.Accordingly, in FIG. 2 there is shown flash lamps 24 electricallyconnected in branch circuits to assure that the branch circuit of eachlamp will become an open circuit upon flashing of the lamp in thatcircuit. The desired mode of operation is carried out starting with theleftmost branch circuit when the lamp in said circuit is flashed andcauses fuse element 26 in the same branch circuit to physicallyinterrupt the circuit path to said lamp while simultaneously opening thecircuit path to the flash lamp in the adjacent branch circuit byactuating the radiation sensitive switch 22 serially connected betweensaid branch circuits to the conductive state. This cooperative action isrepeated between successive adjacent branch circuits until all fourlamps in a group have been fired from simultaneous actuation ofadjoining switch and fuse elements with radiation being emitted from anadjacent flash lamp.

As previously indicated, a fuse element for operation in the abovedescribed lamp firing circuitry can simply comprise a more narrow widthof the circuit pattern comprising an organic resin matrix havingparticulated electrically conductive material and a radiation absorbingmaterial dispersed therein than the width of a current pattern elsewherein the electrical circuit and which is located where said dielectricsubstrate has a reduced thickness relative to the remaining thickness ofthe dielectric substrate of the circuit board member. Suitable fuseelements constructed in this manner are thereby modifications of theconductive ink compositions disclosed in the aforementioned 3,968,056and 3,988,647 patents wherein a radiation absorbing material has beendispersed in the liquid ink to enhance thermal decomposition whenactuated by radiation being emitted from the operatively associatedflash lamps. Accordingly, an illustrative circuit ink composition canhave in proportions by weight 0.5 parts hydrocarbon soluble dye, 32.5parts of an ultraviolet curable polyester resin, and 67.0 parts silvercoated glass spheres. When cured to the solid state at ordinarytemperatures the more narrow width fuse element portions of said circuitboard pattern effectively interrupts the circuit path when actuated byan adjoining flash lamp to produce physical separation of theincorporated electrically conductive filler particles. The indentationsor reduced thickness portions of the circuit board members at fuselocations serve to reduce dissipation of the absorbed thermal and/orlight energy when the fuse elements are actuated in this manner as wellas enhance localized melting or decomposition of the circuit boardmember at the fuse locations causing an actual opening in saidunderlying substrate.

While preferred embodiments of the invention have been shown anddescribed, various other embodiments and modifications thereof willbecome apparent to persons skilled in the art. For example, the desiredcircuit interruption can also be achieved in other firing circuitconfigurations such as one wherein the switch and fuse elements are bothdisposed appropriately in branch circuits with the flash lamps toprovide the desired firing sequence. It is therefore intended to limitthe present invention only by the scope of the following claims.

What we claim as new and desire to secure by Letters Patent of theUnited States is:
 1. In a circuit board for a photoflash lamp arrayhaving a lamp firing circuit pattern deposited on a dielectric substrateand radiation sensitive switches connected in said circuitry forsequential firing of a plurality of flash lamps connected thereto, theimprovement wherein said circuit pattern comprises an organic resinmatrix having particulated electrically conductive material and aradiation absorbing material dispersed therein, said circuit patternfurther including radiation sensitive fuse elements located at positionswhere the dielectric substrate has a reduced thickness relative to theremaining thickness of said dielectric substrate and with said fuseelements being formed by providing a more narrow width of said circuitpattern at the fuse locations than the remaining width of said circuitpattern, each of said flashlamps and radiation fuse elements beingconnected in branch circuits having the radiation sensitive switchesconnected therebetween.
 2. A circuit board as in claim 1 wherein theradiation absorbing material dispersed in the circuit pattern is a dye.3. A circuit board as in claim 1 wherein the particulated electricallyconductive material comprises metal coated glass spheres.
 4. A circuitboard as in claim 3 wherein the particulated electrically conductivematerial further contains metal flakes.
 5. A circuit board as in claim 1wherein the radiation sensitive switches and the radiation sensitivefuse elements are located adjacent said flash lamps for simultaneousactivation of an adjoining switch and fuse element.
 6. In a circuitboard for a photoflash array having a lamp firing circuit patterndeposited on a dielectric substrate and radiation sensitive switchesconnected in said circuitry for sequential firing of a plurality offlash lamps connected thereto, the improvement wherein said circuitpattern comprises radiation cured organic resin matrix having aparticulated electrically conductive material selected from the groupconsisting of a particulated electrically conductive metal or aparticulated electrically conductive metal containing material,including mixtures thereof, and with no more than about 15% by weight ofsaid particulated electrically conductive material having an aspectratio of diameter to thickness of a value greater than 20 beingdispersed in said organic resin matrix along with a radiation absorbingmaterial, said circuit pattern further including radiation sensitivefuse elements located at positions where the dielectric substrate has areduced thickness relative to the remaining thickness of said dielectricsubstrate and with said fuse elements being formed by providing a morenarrow width of said circuit pattern at the fuse locations than theremaining width of said circuit pattern, each of said flashlamps andradiation fuse elements being connected in branch circuits having theradiation sensitive switches connected therebetween.
 7. A circuit boardas in claim 6 wherein the radiation absorbing material dispersed in thecircuit pattern is a dye.
 8. A circuit board as in claim 6 in which atleast 85% by weight of said particulated electrically conductivematerial is constituted by metal coated glass spheres.
 9. A circuitboard as in claim 6 in which at least about 40% and not more than about90% by weight of said circuit pattern is said particulated electricallyconductive material.
 10. A circuit board as in claim 6 wherein theradiation sensitive switches and the radiation sensitive fuse elementsare located adjacent said flash lamps for simultaneous activation of anadjoining switch and fuse element.